Constrained Sintering of Patterned Ceramic Films on Stiff Substrates

Constrained sintering is a common step during the fabrication of devices such as integrated circuit boards, ceramic gas sensors or low temperature co-fired ceramic (LTCC) modules. Due to the layered nature of those components, special challenges arise since in-plane stresses due to the geometrical constraint imposed by the substrate may cause damage and loss of functionality. Possible damage scenarios are delamination along the film-substrate interface, cracking of preexisting flaws or mud-cracking. The degree of constraint and thus the magnitude of the stresses depend on the nature of the interface, which in turn is influenced by factors such as material, substrate roughness and feature size. Due to the high cost of prototyping, prediction of the sintering behavior of different features by simulation methods is desirable. Like this, near-net shape fabrication of patterned films is achievable. In this work, patterned alumina features with thicknesses ranging from 5µm to 32µm and lateral dimensions between 10µm and 500µm have been fabricated using micro-molding in capillaries. Stripes with and without intentionally introduced flaws as well as ring structures are available. The influence of the substrate properties are studied by comparison of smooth and rough as well as plain and platinum coated sapphire substrates. Experiments are repeated with a liquid phase sintering aid in case of the smooth sapphire substrates. Vertical and lateral strain data are analyzed using white light interferometry as well as scanning electron microscopy. Stripe cross sections display lateral and vertical density gradients, leading to a high density skin at all free surfaces that encloses a porous area near the substrate. Elongated pores oriented perpendicular to the substrate are found in the stripe center, whereas pores near the edge are less oriented. Edge delamination is present in all materials and increases with decreasing thickness-to-width ratio of the ceramic stripes. Lateral and vertical sintering strains are characterized; the former are found to be strongly dependent on feature size and achieve values close to the vertical strains in the case of narrow features, whereas strong enhancement of the vertical strains and near-zero lateral strains are characteristic of the wider stripes. Substrate roughness leads to decreasing lateral strains, reduced delamination and lower densification. An intermediate platinum layer allows for larger lateral strains despite lower delamination by accommodation of lateral stresses. The degree of constraint is quantified by using the vertical-to-lateral strain ratio. All studied materials show a transition from quasi-free sintering behavior (ratios close to unity) at high thickness-to-width ratios and perfect constraint (ratios above unity) at low ratios. The transition occurs at lower ratios when metallic interlayers are present, thus reducing the degree of constraint. When a liquid phase sintering aid is present, quasi-free sintering behavior is found at lower thickness-to-width ratios than in case of solid state sintered structures. The above mentioned strain data are used to calculate the interface friction coefficient. The latter is found to follow model predictions and is used to predict the shrinkage behavior of ring structures. In addition, intentionally introduced flaws show cracking during sintering. Their propensity to crack is greater in narrow stripes; in addition, the proximity of the stripe edge results in oriented cracks perpendicular to the stripe edge. Wide stripes experience radial cracking. The degree of cracking depends on the lateral strain experienced by the overall stripe, which in turn is determined by material and substrate roughness. In-situ environmental scanning electron microscope pictures prove that crack propagation occurs via micro-crack coalescence in oriented cracks. The experimental data in this work have been placed at the disposal of simulation experts who have provided simulations using the discrete element and finite element methods. These simulation results reproduce important experimental findings such as lower sintered density in stripes on rough substrates as well as delamination and shape distortion during sintering